The macaque's retina contains several distinct functional classes of ganglion cell, at least two of which project to the lateral geniculate nucleus (LGN) and terminate in different laminae. The segregation of pathways, preserved throught the LGN, becomes obscured in the striate cortex. The experiments proposed here are intended to answer the following questions: (i) which cortical neurons are driven by the different classes of input, and how does their behavior reflect the distinctive inputs; (ii) what are the visual roles of these "parallel pathways". The plan of investigation is first to clarify some properties of the distinctive classes of LGN neuron in monkey (sensitivity to spatio-chromatic contrast; sensitivity to stimulation remote from the classical receptive field; whether or not the neurons receive inputs from rods) so that the inputs to cortical neurons can be better specified. The second stage is to link cortical neurons to their different classes of inputs by the quantitative analysis of receptive field properties in cortex to reveal which classes of LGN neurons could, and which could not, provide the input to the cortical cell. Sinusoidal grating patterns whose luminance and color can be modulated in space and time will be the principal stimuli, because they are potent, they faciliate the application of Fourier techniques to characterize the underlying structure of receptive fields, and they provide results that can be compared with psychophysical measurements of human visual sensitivity. The third component of the project is to measure the "thresholds" of individual neurons (i.e., to find the stimuli that evoke weak, reliable, responses) so that the sensitivities of neurons can be compared with the sensitivities of human observers. It should then be possible to establish which neurons might, and which could not, underlie different aspects of human vusual performance. Some measurements of sensitivities of individual neurons will be undertaken in the awake, behaving, monkey to establish whether or not the properties of cells differ in important ways from those found in the anaesthetized animal.